The present invention relates to variable resistor circuits, the resistance of which is controlled by an electric signal, more particularly a voltage.
A variable resistor circuit is generally constituted by a dipole, the resistance or impedance of which takes a determined value as a function of an electric variable, for example a voltage, applied across the terminals of the circuit.
Variable resistor circuits are frequently used in many electronic devices. They are used, for example, for controlling a current or a voltage, for setting a gain or attenuation, or for adjusting filter parameters.
A variable resistor circuit can be formed by using a MOS-type transistor. In a MOS transistor, the resistance between the two main electrodes varies as a function of the voltage applied to the control electrode. In other words, the resistance between the drain and source of a MOS transistor varies as a function of the voltage applied between the gate and the source. Therefore, a simple MOS transistor is liable to constitute a satisfactory voltage-controlled variable resistance. Such a variable resistor circuit properly operates only if the MOS transistor constantly remains in the linear area of the drain-source current voltage characteristic. If the drain-source voltage exceeds a determined value, depending on the physical characteristics of the MOS transistor, the circuit is no longer in the linear area of its characteristic but enters the saturation area wherein the drain-source current does not proportionally varies with the drain-source voltage. Hence, a MOS transistor can be used for constituting a voltage-controlled resistor only if the maximum voltage liable to be applied across the terminals of the variable resistor, that is, between the drain and source, remains lower than the voltage corresponding to the start of the MOS transistor saturation.
However, in many conventional electronic circuits, a large number of different functions is implemented in a single integrated circuit. Particularly, it is presently possible to interconnect in a same chip bipolar transistors and MOS transistors. In some applications, bipolar transistors are to withstand high voltages, for example about 50 volts, and MOS transistors are to withstand substantially lower voltages, for example about 5 volts. It will be easily understood that in such an integrated circuit it will not be possible to use a MOS transistor for constituting a variable resistor if the latter is designed to be connected to a portion of the circuit constituted by bipolar transistors liable to supply substantially higher voltages.
One of the aims of the invention is to solve this problem.